Unfortunately, I can't give you the exact solution to fix it, but I have some suggestions. I had the same kind of problems while setting up my own mining rig, so wish you good luck with that.

Check if the problem is caused by specific PCI-E slots. If not, you should check the Risers to verify that they are all working without any errors. So far so good?, then I believe you have something wrong with your softwares (OS, Drivers, etc.), which is better because software-sided problems are eaiser to fix. I think you can test it out by using Simplemining OS, since it doesn't require any software setups, tweaks and etc, it should be eaiser to check if all cards are recognized by your os and working well. Other than that, you can uninstall your graphic card drivers with ddu and plug them one by one (restart per every card plug.). If none of these are working for you, as a last resort, you can check your bios settings. check youtube videos to see if you have a missing configuration or something wrong with your settings.

The information you've provided is so little, so all I can do is to give you some advice and make assumptions.